ured himself a second glass of sherry, took a sip, and rolled
it carefully over his tongue.

The Centaurus Mystery. That's what the explorers had called it back in
2041, nearly a century and a half before, when they'd found the great
city on one of the planets of the Alpha Centaurus system. Man's first
interstellar trip had taken nearly five years at sublight velocities,
and _bing!_--right off the bat, they'd found something that made
interstellar travel worthwhile, even though they'd found no planet in
the Alpha Centaurus system that was really habitable for man.

They'd seen it from space--a huge domed city gleaming like a great gem
from the center of the huge desert that covered most of the planet. The
planet itself was Marslike--flat and arid over most of its surface, with
a thin atmosphere high in CO_2 and very short on oxygen. The city showed
up very well through the cloudless air.

From the very beginning, it had been obvious that whoever or whatever
had built that city had not evolved on the planet where it had been
built. Nothing more complex than the lichens had ever evolved there, as
thousands of drillings into the crust of the planet had shown.

Certainly nothing of near-humanoid construction could ever have come
into being on that planet without leaving some trace of themselves or
their genetic forebears except for that single huge city.

How long the city had been there was anyone's guess. A thousand years? A
million? There was no way of telling. It had been sealed tightly, so
none of the sand that blew across the planet's surface could get in. It
had been set on a high plateau of rock, far enough above the desert
level to keep it from being buried, and the transparent dome was made of
an aluminum oxide glass that was hard enough to resist the slight
erosion of its surface that might have been caused by the gentle, thin
winds dashing microscopic particles of sand against its smooth surface.

Inside, the dry air had preserved nearly every artifact, leaving them as
they had been when the city was deserted by its inhabitants at an
unknown time in the past.

That's right--deserted. There were no signs of any remains of living
things. They'd all simply packed up and left, leaving everything behind.

Dating by the radiocarbon method was useless. Some of the carbon
compounds in the various artifacts showed a faint trace of radiocarbon,
others showed none. But since the method depends on a knowledge of the